Mitochondrial and peroxisomal beta-oxidation enzymes degrade saturated and unsaturated fatty acids by sequential removal of two-carbon units from Coenzyme A (CoA)-activated fatty acids. The main beta-oxidation pathway degrades both saturated and unsaturated fatty acids while the auxiliary pathway performs additional steps required for the degradation of unsaturated fatty acids.
The pathways of mitchondrial and peroxisomal beta-oxidation use similar enzymes, but have different substrate specificities and functions. Mitochondria oxidize short-, medium-, and long-chain fatty acids to produce energy for cells. Mitochondrial beta-oxidation is a major energy source for cardiac and skeletal muscle. In liver, it provides ketone bodies to the peripheral circulation when glucose levels are low as in starvation, endurance exercise, and diabetes. (Eaton, S. et al. (1996) Biochem. J. 320:345-357.) Peroxisomes oxidize medium-, long-, and very-long-chain fatty acids, dicarboxylic fatty acids, branched fatty acids, prostaglandins, xenobiotics, and bile acid intermediates. The chief roles of peroxisomal beta-oxidation are to shorten toxic lipophilic carboxylic acids to facilitate their excretion and to shorten very-long-chain fatty acids prior to mitochondrial beta-oxidation. (Mannaerts, G. P. and Van Veldhoven, P. P. (1993) Biochimie 75:147-158.)
The auxiliary beta-oxidation enzyme 2,4-dienoyl-CoA reductase catalyzes the following reaction: EQU trans-2, cis/trans-4-dienoyl-CoA+NADPH+H.sup.+ trans-3-enoyl-CoA+NADP.sup.+
This reaction removes even-numbered double bonds from unsaturated fatty acids prior to their entry into the main beta-oxidation pathway. (Koivuranta, K. T. et al. (1994) Biochem. J. 304:787-792.) The enzyme may also remove odd-numbered double bonds from unsaturated fatty acids. (Smeland, T. E. et al. (1992) Proc. Natl. Acad. Sci. USA 89:6673-6677.)
Rat 2,4-dienoyl-CoA reductase is located in both mitochondria and peroxisomes. (Dommes, V. et al. (1981) J. Biol. Chem. 256:8259-8262.) Two immunologically different forms of rat mitochondrial enzyme exist with molecular masses of 60 kDa and 120 kDa. (Hakkola, E. H. and Hiltunen, J. K. (1993) Eur. J. Biochem. 215:199-204.) The 120 kDa mitochondrial rat enzyme is synthesized as a 335 amino acid precursor with a 29 amino acid N-terminal leader peptide which is cleaved to form the mature enzyme. (Hirose, A. et al. (1990) Biochim. Biophys. Acta 1049:346-349.) A human mitochondrial enzyme 83% similar to rat enzyme is synthesized as a 335 amino acid residue precursor with a 19 amino acid N-terminal leader peptide. (Koivuranta, supra.) These cloned human and rat mitochondrial enzymes function as homotetramers. (Koivuranta, supra.) A Saccharomyces cerevisiae peroxisomal 2,4-dienoyl-CoA reductase is 295 amino acids long, contains a C-terminal peroxisomal targeting signal, and functions as a homodimer. (Coe, J. G. S. et al. (1994) Mol. Gen. Genet. 244:661-672; and Gurvitz, A. et al. (1997) J. Biol. Chem. 272:22140-22147.) All 2,4-dienoyl-CoA reductases have a fairly well conserved NADPH binding site motif of sequence -h-X-h-X-Gly-X-Gly-X-X-Gly-X-X-X-h-X-X-h- . . . Asp/Glu-, where h=hydrophobic amino acid residue and X=any amino acid residue. (Koivuranta, supra.)
The main pathway beta-oxidation enzyme enoyl-CoA hydratase catalyzes the following reaction: EQU 2-trans-enoyl-CoA+H.sub.2 O+3-hydroxyacyl-CoA
This reaction hydrates the double bond between C-2 and C-3 of 2-trans-enoyl-CoA, which is generated from saturated and unsaturated fatty acids. (Engel, C. K. et al. (1996) EMBO J. 15:5135-5145.) This step is downstream from the step catalyzed by 2,4-dienoyl-reductase. Different enoyl-CoA hydratases act on short-, medium-, and long-chain fatty acids. (Eaton, supra.) Mitochondrial and peroxisomal enoyl-CoA hydratases occur as both mono-functional enzymes and as part of multi-functional enzyme complexes. Human liver mitochondrial short-chain enoyl-CoA hydratase is synthesized as a 290 amino acid precursor with a 29 amino acid N-terminal leader peptide. (Kanazawa, M. et al. (1993) Enzyme Protein 47:9-13; and Janssen, U. et al. (1997) Genomics 40:470-475.) Rat short-chain enoyl-CoA hydratase is 87% identical to the human sequence in the mature region of the protein and functions as a homohexamer. (Kanazawa, supra; and Engel, supra) A mitochondrial trifunctional protein exists that has long-chain enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase, and long-chain 3-oxothiolase activities. (Eaton, supra.) In human peroxisomes, enoyl-CoA hydratase activity is found in both a 327 amino acid residue mono-functional enzyme and as part of a multi-functional enzyme, also known as bifunctional enzyme, which possesses enoyl-CoA hydratase, enoyl-CoA isomerase, and 3-hydroxyacyl-CoA hydrogenase activities. (FitzPatrick, D. R. et al. (1995) Genomics 27:457-466; and Hoefler, G. et al. (1994) Genomics 19:60-67.) A 339 amino acid residue human protein with short-chain enoyl-CoA hydratase activity also acts as an AU-specific RNA binding protein. (Nakagawa, J. et al. (1995) Proc. Natl. Acad. Sci. USA 92:2051-2055.) All enoyl-CoA hydratases share homology near two active site glutamic acid residues, with 17 amino acid residues highly conserved. (Wu, W.-J. et al. (1997) Biochemistry 36:2211-2220.)
Inherited deficiencies in mitochondrial and peroxisomal beta-oxidation enzymes are associated with severe diseases, some of which manifest themselves soon after birth and lead to death within a few years. Mitochondrial beta-oxidation associated deficiencies include, e.g., carnitine palmitoyl transferase and carnitine deficiency, very-long-chain acyl-CoA dehydrogenase deficiency, medium-chain acyl-CoA dehydrogenase deficiency, short-chain acyl-CoA dehydrogenase deficiency, electron transport flavoprotein and electron transport flavoprotein:ubiquinone oxidoreductase deficiency, trifunctional protein deficiency, and short-chain 3-hydroxyacyl-CoA dehydrogenase deficiency. (Eaton, supra.) Mitochondrial trifunctional protein (including enoyl-CoA hydratase) deficient patients have reduced long-chain enoyl-CoA hydratase activities and suffer from non-ketotic hypoglycemia, sudden infant death syndrome, cardiomyopathy, hepatic dysfunction, and muscle weakness, and may die at an early age. (Eaton, supra.) A patient with a deficiency in mitochondrial 2,4-dienoyl-CoA reductase was hypotonic soon after birth, had feeding difficulties, and died at four months from respiratory acidosis. (Roe, C. R. et al. (1990) J. Clin. Invest. 85:1703-1707.)
Defects in mitochondrial beta-oxidation are associated with Reye's syndrome, a disease characterized by hepatic dysfunction and encephalopathy that sometimes follows viral infection in children. Reye's syndrome patients may have elevated serum levels of free fatty acids. (Cotran, R. S. et al. (1994) Robbins Pathologic Basis of Disease, W. B. Saunders Co., Philadelphia, Pa., p.866.) Patients with mitochondrial short-chain 3-hydroxyacyl-CoA dehydrogenase deficiency and medium-chain 3-hydroxyacyl-CoA dehydrogenase deficiency also exhibit Reye-like illnesses. (Eaton, supra; and Egidio, R. J. et al. (1989) Am. Fam. Physician 39:221-226.)
Inherited conditions associated with peroxisomal beta-oxidation include Zellweger syndrome, neonatal adrenoleukodystrophy, infantile Refsum's disease, acyl-CoA oxidase deficiency, peroxisomal thiolase deficiency, and bifunctional protein deficiency. (Suzuki, Y. et al. (1994) Am. J. Hum. Genet. 54:36-43; Hoefler, supra) Patients with peroxisomal bifunctional enzyme, including enoyl-CoA hydratase, deficiency suffer from hypotonia, seizures, psychomotor defects, and defective neuronal migration; accumulate very-long-chain fatty acids; and typically die within a few years of birth. (Watkins, P. A. et al. (1989) J. Clin. Invest. 83:771-777.)
Peroxisomal beta-oxidation is impaired in cancerous tissue. Although neoplastic human breast epithelial cells have the same number of peroxisomes as do normal cells, fatty acyl-CoA oxidase activity is lower than in control tissue. (el Bouhtoury, F., et al. (1992) J. Pathol. 166:27-35.) Human colon carcinomas have fewer peroxisomes than normal colon tissue and have lower fatty-acyl-CoA oxidase and bifunctional enzyme (including enoyl-CoA hydratase) activities than normal tissue. (Cable, S., et al. (1992) Virchows Arch. B Cell Pathol. Incl. Mol. Pathol. 62:221-226.)
The discovery of new human fatty acid beta-oxidation enzymes and the polynucleotides encoding them satisfies a need in the art by providing new compositions which are useful in the diagnosis, treatment, and prevention of genetic disorders, neuronal disorders, cancer, infectious diseases, liver disorders, and cardiac and skeletal muscle disorders.